The planet’s large metal core is slowly cooling. As the core cools, the whole planet contracts and the surface forms wrinkle‑like lines.

Researchers used pictures taken by a NASA spacecraft to count these wrinkle lines and to draw a map of the planet’s surface.

A joint European‑Japanese mission, the BepiColombo spacecraft, should start orbiting Mercury in November. The mission should show how much smaller Mercury has become, a scientist said.